"There was this idea that we scan the dream image, or the mental image when we dream," said study co-author Yuval Nir, a sleep researcher and neuroscientist at Tel Aviv University in Israel. "But it's been very difficult to prove it."

What dreams are made of

The mystery of why humans dream has long fascinated scientists and philosophers. Are dreams hints of a person's deepest, most subconscious longings, as Sigmund Freud proposed? Are they the way the brain works out real-life problems? Or are dreams the day's mental debris, which the brain sweeps up every night as a person slumbers?

Yet despite years of study, scientists had few answers about this mysterious space between sleep and wakefulness. For about 60 years, researchers knew that people dreamed during the rapid eye movement (REM) phase of sleep. Many hypothesized that the darting of the eyes corresponded to people "seeing" images in their dreams, but there was no way to know for sure, Nir said.

But then Nir and his colleagues found a unique way to test out the idea. Some people with epilepsy continue to have many seizures despite medication, and instead must have electrodes implanted in their brains. The electrodes send small electrical jolts to the parts of the brain responsible for the seizures, and often the treatment completely eliminates the seizures, Nir said.

But the electrodes can also detect and record the electrical firing from individual neurons, or brain cells.

Sight in sleep

So Nir, Dr. Itzhak Fried, a cognitive neurophysiologist at UCLA, and their colleagues asked 19 people who were having brain implants for epilepsy if they would also be willing to have some of their brain activity recorded while they slept.

The brain recordings revealed that every time the dreamers moved their eyes, neurons in the medial temporal lobe of the brain showed a burst in activity, the researchers reported Aug. 11 in the journal Nature Communications.

In past studies, Fried had found that the medial temporal lobe lit up when people looked at images of famous people and places, such as the Golden Gate Bridge or Jennifer Aniston; simultaneously, they also made eye movements similar to those found during REM sleep. In 2010 Fried and his colleagues published a paper in the journal Nature showing that neurons in this brain region were under voluntary, conscious control when people were awake.

The new study finally confirms the long-held hypothesis that eye movements during REM sleep correspond to people visualizing imagery in their dreams, said Dr. J. Allan Hobson, a psychiatrist and sleep researcher at Harvard Medical School in Massachusetts, who was not involved in the current study.

Many other parts of the brain were also active during REM sleep, and the team still isn't sure how those brain regions are involved in dreaming, Nir said.

Theoretically, researchers could one day read someone's dream activity using brain-firing data, determining whether the person was visualizing, say, an adorable kitten versus a scary monster. To do that, researchers would have to record someone's brain cell-firing signals during sleep, and then wake the person up to ask about the dream that individual was having. They would also need to collect data on the brain's electrical firing signals when the person viewed those same images during wakefulness.

"This is the Holy Grail that we'd like to do someday," Nir told Live Science. "The only thing is, it's quite challenging to collect dream reports in this particular setting."

After all, people undergoing brain surgery are already stressed about their health and are distracted by other concerns; waking them up in the middle of the night would be too intrusive, Nir said.

Purpose of dreams?

Though the study focuses on the mechanics of dreaming, it has other implications, Hobson said.

"I think this data is just another nail in the coffin in the idea that dreaming is a sort of symbolic distortion of past activity," Hobson said, referring to Freudian ideas that dreams have symbolic significance that must be interpreted.

Instead, because the same types of visual activity are under a person's conscious control when the individual is awake, the dream-state brain-cell firing "shows very clearly that dreaming is a form of consciousness," Hobson said. "You might call it an altered state of consciousness, an alternative state of consciousness."

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